An optical densitometer is a device used to determine the optical density of printed ink films. A densitometer normally contains four optical filters each matched to one of the four standard printing ink colors, cyan, magenta, yellow and black.
The densitometer is an indispensable tool to the pressmen who operate four color lithographic presses. It provides accurate, repeatable and objective readings of optical density of solid patches of ink, halftone patches and overprinted patches. Density readings taken on these patches are used to adjust inking levels on the press and to adjust other printing parameters such as dampening solution levels and roller nip pressures.
The densitometer user normally performs a referencing procedure before measuring the density of printed ink. The optical density of a patch of the unprinted substrate is read through each of the four filters. These densities are designated as reference densities by the user and stored as such in the densitometer's memory. If the readings are made but not designated as reference readings, they will not be stored as such and the user will subsequently read densities erroneously believed to be referenced to the unprinted substrate. Actual density readings require the user to first identify the color of the target patch and decide whether the target is a solid, halftone or overprinted patch. The user must also correctly match the target patch color to the filter chosen to read the optical density. An error at any point in this procedure will, of course, yield a useless density reading since it is misidentified. By way of example, it is possible to confuse cyan and blue or red and magenta.
While taking readings during a press run, the pressman must concentrate on performing the correct sequence of events. He is necessarily distracted from the more important business of monitoring the behavior of the press. Also, if the densitometer is not easily portable, the pressman's movement is restricted by the length of the densitometer's line cord. A densitometer that cannot be held in one hand when operated is less convenient to use than a hand-held version of the same device.
It is desirable to eliminate user error to the extent possible, since user error results in wasted printing material and lost time. Thus, it would be highly desirable to provide a densitometer capable of automatically performing decision-making tasks. Such tasks include (i) the ability to recognize unprinted substrate and read and store all four filter position densities as reference densities, (ii) the ability to identify the color of the target being read, match the color to the appropriate filter and display the appropriate density, (iii) the ability to distinguish among a group of solid patches, (iv) the ability to distinguish between solids and halftones, solids and overprints and halftones and overprints, (v) the ability to determine and display percent trap of an overprint and (vi) the ability to determine and display percent dot area of a halftone.
A densitometer capable of performing the above mentioned decision-making tasks in an unaided manner would advantageously reduce user error and free the pressman to concentrate on operating the press rather than the densitometer. A battery operated, hand-held densitometer which is capable of performing the above tasks would be even more desirable since it could be taken anywhere in the pressroom area to read optical density.
Gretag introduced a hand-held battery operated densitometer, the Gretag D-1, in 1966. This densitometer does not read all four filter positions automatically for each sample and does not perform the decision-making tasks described above.
Macbeth.sup..RTM. introduced the TDA-1000 densitometer in 1970. This device provided four sequential readings of each sample. It was not a portable device, however, and did not perform decision making tasks. In 1977, Macbeth.sup..RTM. introduced the On-Press Color Monitor, an on-line densitometer which took four simultaneous readings of optical density on four successively positioned color patches. The color and relative position of each patch was preprogrammed into the memory of the on-line operating system. The densitometer consisted of four distinct sets of light sources and filtered detectors. This device was not portable and did not perform the type of decision-making tasks described above. In 1981, Macbeth.sup..RTM. introduced a version of the On-Press Color Monitor using a single light source and a fiber optic probe which was a single bundle at the point where light reflected from the sample was collected and which was then separated into four bundles (quadrifurcated). Each of these bundles passed a portion of the reflected light through a blue, green, red or visual filter, respectively, to one of the four detectors. This device was not portable and could not perform the types of decision-making tasks described above.
U.S. Pat. No. 4,239,393 issued to Tobias in 1980 describes a densitometer containing a rotating filter wheel having red, green and blue filters which make it possible to read three densities automatically for each sample. This device is not portable, does not provide a separate filter matched to black ink, and is not capable of performing decision-making tasks.
In 1986, X-Rite introduced the X-Rite 408 Color Reflection Densitometer. This device is battery operated and hand-held. It uses a rotating filter wheel to read all four filter positions for each sample. It displays the largest optical density reading of the four filters. This device does not distinguish between solids and halftones, solids and overprints or overprints and solids. Nor does it recognize an unprinted substrate as a reference without operator instruction.
Therefore, it is an object of the present invention to provide a portable hand-held densitometer.
Another object of the present invention is to provide a densitometer which automatically recognizes unprinted substrate, overprints, solids and halftones.
Another object of the present invention is to provide a densitometer which automatically selects and displays appropriate density measurements.
A further object of the present invention is to provide a densitometer which automatically selects the appropriate density measurements and calculates and displays the percent trap of an overprint.
Yet a further object of the present invention is to provide a densitometer which automatically selects the appropriate density measurements and calculates and displays percent dot area.
These and other highly desirable and remarkable results are accomplished by the present invention in a portable, hand-held densitometer which automatically determines type of the target being measured, stores pertinent measured parameters therefrom and, where appropriate, determines and displays vital information parameters such as percent trap and percent dot area for use by the pressman in adjusting and controlling the printing press.
Objects and advantages of the invention are set forth in part herein and in part will be obvious herefrom, or may be learned by practice with the invention, the same being realized and attained by means of the instrumentalities and combinations pointed out in the appended claims.
The invention consists in the novel parts, constructions, arrangements, combinations, steps and improvements herein shown and described.